1. Field of the Invention
The present invention relates to electro-optic light valves and an electronic control system therefore. Particularly the invention relates to an electronic light valve apparatus suited for use on a photographic film printer and a control system for regulating the light valves in accordance with programmed information.
2. Description of the Prior Art
The use of light valves in photographic film printing to adjust for color balance and hue corrections is well known in the art. Color balance correction is often required during a printing process to accommodate for the various lighting conditions used for photographing original scenes during the production of a movie. Since some scenes are often shot outdoors under natural lighting conditions, and other scenes are shot indoors under various artificial lighting conditions, the color rendition of actor's clothing and the like will appear to be different in various scenes. Therefore, customarily, during the printing operation, when the preprint film is printed onto raw stock, the color content of the printing beam is varied in order to achieve a motion picture movie print in which the color changes are not noticeable to the viewer.
An additive color system is provided on the photographic film printer to vary the color content of the printing beam. In one type of apparatus, a substantially white light beam is separated into three substantially monochromatic light beams, i.e., the primary colors red, blue and green, by a plurality of dichroic mirrors. A light valve is positioned within each of the monochromatic light beams to adjustably control the cross sectional area of the beam to compensate for the varying lighting conditions under which the scenes were photographed and a second plurality of (dichroic) mirrors are used to recombine the regulated monochromatic light beams into a final modulated color content printing beam for exposing the raw stock film.
One type of color light valve suitable for this purpose is shown in U.S. Pat. No. 3,454,328 filed Jan. 19, 1966 and assigned to the assignee of the present invention.
Additionally, in modern motion picture film production, it is often desirable to provide an exposure control device on the additive color system to permit the use of special effects such as fades, lap dissolves, etc. Conventional exposure control devices provide a shutter across the printing light beam which reduces the intensity of the total light beam without affecting the color correction achieved by the light valves.
Prior to exposure of the raw stock, the preprint film is visually inspected by a skilled operator for the purposes of determing the amount of correction desired for each scene. According to conventional practice, lighting correction for each scene is recorded by punching a paper tape. During printing, the photographic film printer reads the punched tape to control the light valves in accordance with the punched information. The position at which desired color changes will be affected is determined by one of three types of cuing systems. The first type of cuing system comprises notches on the preprint film which are sensed by the photographic film printer as the film passes therethrough, to initiate a color change at the beginning of a scene. In a second type of system, RF patches are secured to the preprint film and are similarly sensed to initiate color changes. In a third, and more efficient system, a frame count cuing system is provided in which a unique film address is associated with each desired color change. In this last system, the frame count cuer constantly monitors the amount of film being transported through the film printer and as predetermined frames reach the printing aperture, the frame count cuer initiates the desired color changes. A suitable frame count cuer is shown in application Ser. No. 509,590 filed Sept. 26, 1974 now U.S. Pat. No. 4,072,419 and assigned to the assignee of the present invention. A suitable fader for use on a photographic film printer is shown in U.S. Pat. No. 4,009,959 filed June 26, 1975, assigned to the assignee of the present invention.
Because of the mechanical elements utilized in the above described prior art devices, certain inherent, mechanical delays and bouncing are caused by, among other factors, the mass and inertial forces of these mechanical elements. Therefore elaborate techniques were devised in order to generate a signal prior to exact position of the desired color change on the preprint film, to accommodate for these mechanical delays. Additionally, previous systems did not permit for color balance corrections during the execution of a special effect, such as a fade or lap dissolve.